1. Field of the Invention
The present invention relates to a power-supply unit for a microwave tube that supplies high tension power to a klystron, a traveling-wave tube and others, particularly relates to a power-supply unit for a microwave tube for preventing the breakdown of a microwave tube when power supply is turned off.
2. Description of the Prior Art
Heretofore, this type of power-supply unit for a microwave tube is disclosed in Japanese published unexamined patent application No. Hei4-129132. FIG. 1 is a block diagram showing a conventional type power-supply unit for a microwave tube. As shown in FIG. 1, a collector power source 61 is connected between a cathode power supply terminal 64 and a collector power supply terminal 65 and supplies voltage to be applied to a cathode electrode to a collector electrode of a microwave tube (not shown) A helix power source 62 is connected between the collector power supply terminal 65 and a helix power supply terminal 66 and supplies a helix electrode of the microwave tube with more potential than potential which the collector power source 61 supplies. A helix current detecting circuit 63 is inserted before the helix power supply terminal 66. The collector power source 61 and the helix power source 62 are respectively formed by a step-up transformer for boosting input voltage so that the voltage is high voltage of 1 kV or more, a switching inverter and a rectifier composed of a diode bridge and a smoothing capacitor for rectifying boosted voltage so that the voltage becomes d.c. voltage, and it is general that primary winding of the step-up transformer and the input side of the switching inverter are common.
Further, a voltage controlling element 68 is connected in parallel with the helix power source 62, in other words, between the collector power supply terminal 65 and the output terminal 67 of the helix power source 62, and the clamp voltage of the voltage controlling element 68 is set to withstand voltage or less between the helix electrode and the collector electrode of the microwave tube connected to the helix power supply terminal 66 and the collector power supply terminal 65.
For impedance between each electrode of the microwave tube viewed from the side of the power-supply unit, in case the respective electrodes of the microwave tube are connected to each power supply terminal of such a power-supply unit, impedance between the collector electrode and the cathode electrode is smaller than impedance between the helix electrode and the cathode electrode. Therefore, in a sequence in which the collector power source 61 and the helix power source 62 are simultaneously turned off, the voltage of a collector approximates the voltage of a cathode earlier than the voltage of a helix and difference between the voltage of the helix and the voltage of the collector is excessive. When the difference exceeds the clamp voltage of the voltage controlling element 68, the voltage controlling element 68 conducts and limits difference in voltage between the helix electrode and the collector electrode so that the difference is a set value or smaller so as to prevent the breakdown of the microwave tube.
However, as withstand voltage of 1 kV or more is required in case a varistor and Zener diode are used for the voltage controlling element 68, multiple elements are required to be connected in series and there is a defect that the unit cannot be miniaturized.
The object of the invention is to provide a power source for a microwave tube in which in a power turn-off sequence, difference in voltage between a helix electrode and a collector electrode of a connected microwave tube can be limited and further, difference in voltage between a cathode electrode and the collector electrode can be limited so as to prevent the breakdown of the microwave tube. Another object is to provide the power source for the microwave tube suitable for miniaturization.
To solve the problem, the invention is based upon a power-supply unit for a microwave tube provided with a helix power supply terminal, a collector power supply terminal and a cathode power supply terminal for supplying power to a helix electrode, a collector electrode and a cathode electrode respectively of the microwave tube and is characterized in that potential dividing means is provided between the helix power supply terminal and the cathode power supply terminal and a potential dividing point of the potential dividing means and the collector power supply terminal are connected via uni-directional conductive means.
The potential dividing means is a capacitor connected in series. Besides, the invention is also characterized in that the uni-directional conductive means is at least one diode the anode of which is connected to the potential dividing point of the potential dividing means and the cathode of which is connected to the collector power supply terminal.
It is desirable that voltage at the potential dividing point of the potential dividing means when the power-supply unit is operated is lower than the voltage of the collector power supply terminal.
The power-supply unit for a microwave tube according to the invention is also characterized in that at least first impedance means and second impedance means are connected in series between the helix power supply terminal and the cathode power supply terminal and the cathode power supply terminal and uni-directional conductive means is connected between a node of the first impedance means and the second impedance means and the collector power supply terminal.
The first and second impedance means can be formed by a capacitor. At least either of the first or second impedance means can be formed by a capacitor and a resistor connected to the capacitor in parallel.
The uni-directional conductive means can be formed by at least one diode the anode of which is connected to a node of the first impedance means and the second impedance means and the cathode of which is connected to the collector power supply terminal. Further, voltage at the node of the first impedance means and the second impedance means can be made lower than the potential of the collector power supply terminal.